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Superluminal Speeds and All That Jazz

by oldman
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oldman
#19
Nov5-08, 07:00 AM
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Quote Quote by MeJennifer View Post
It is absolutely true that in the expansion phase of an FRLW spacetime one can say with equal validity that all clocks speed up instead of all distances increase. Those statements are equivalent in GR.......
I certainly don't mean to insist on using the FRLW metric, although it does seem to me to look like a good approximation to try first. Neither do I like labelling what is going on in our evolving universe "an expansion phase". A pox on the word expansion! But perhaps like you, I'm still somewhat sceptical of the kludges that have been introduced to fix various problems.

Although I may well end up believing in fairies, as you put it, a small niggle keeps scratching at the back of my mind --- is it possible that something else is going on, something that changes the ratio of metric coefficients, something that looks very like expansion, or clocks speeding up --- something really wild like the relative unfolding of the dimensions we are so familiar with, something no one has yet explored or tried to model, what with everybody being so obsessed with how well 'expansion' seems to work; up to the point where kludges have to be brought in?

Mostly I keep such wild thoughts to myself, but they break out now and then. Nuff said.
marcus
#20
Nov5-08, 02:01 PM
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Quote Quote by oldman View Post
...Neither do I like labelling what is going on in our evolving universe "an expansion phase"...


We are in a phase of widespread increasing distances between objects at CMB rest, according to pattern called Hubble law. So what better do you propose to call it? Would you feel comfortable saying we are in a:

1. Hubble phase, or Hubble law phase. (accurate, but too technical to communicate to many listeners)

2. increasing-distances phase (accurate, but too many syllables).

It is easier to say expansion (3 syllables) than increasing-distances (6 syllables). So what's wrong? As long as people don't misunderstand and over-extend the analogy with material substance. Aren't you being a bit fussy about words here?

The basic point is you either buy GR or you offer a better mathematical model of how geometry evolves. If you buy GR then you have no right to expect distances to stay the same. In fact you expect us quite possibly to be in an increasing-distances phase. And the kicker is the idea of being collectively at rest with respect to the matter in the early universe---at rest relative CMB.
Modulo small proper motions and possibly some drift, but on the whole a remarkable collective stationarity.

Well these are the most obvious features of the universe. What words would you like better? I'm flexible about words as long as the mathematical model clearly underlies them. Interpretation is just verbal trimmings. So as long as the words are easy to say, and fairly descriptive, I'm cool.

So what words do you like?
oldman
#21
Nov6-08, 12:30 AM
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Quote Quote by marcus View Post
So what words do you like?
I was replying specifically to MeJennifer and her use of the word "expansion phase" here. You are talking about the consensus model, which she doesn't seem to accept. When talking of this model "expansion" is of course quite acceptable as a description of consensus. Anything else would be clumsy, as you say.

However I'm not yet entirely convinced that the consensus interpretation is correct. I do "buy GR" and I don't "expect distances to stay the same". But, on the other hand, you don't have the right to expect "clocks to always run at the same rate" . All we do know for certain is that physics is invariant in time and space (consequently that GR is a most elegant and satisfying description of both gravity and of this invariance), and that there is a redshift. The rest is interpretation --- which may well have been correctly made --- circumstantial evidence indeed suggest that this is so. But people are fallible, and the consensus interpretation does involve a clutch of ad hoc kludges.

I therfore cavil at certainties, as when you write "We are in a phase of widespread increasing distances between objects at CMB rest, according to pattern called Hubble law." Yes, it looks as if we are, but you write as if this were dogma.

I do like "the idea of being collectively at rest with respect to the matter in the early universe---at rest relative (to the) CMB." Sorts the ether nonsense out nicely.
MeJennifer
#22
Nov6-08, 01:24 AM
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Quote Quote by oldman View Post
...at rest relative (to the) CMB.
I understand what people mean by it but being at rest relative to radiation is just nonsense. Radiation travels at light speed and nothing can be at rest relative to light speed.
mysearch
#23
Nov6-08, 06:04 AM
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The concept of the CMB rest frame seems to raise some interesting issues. As I understand it, the CMB rest frame is not actually making any comparison to a relative velocity to radiation, but rather two events both associated with CMB decoupling.

On the basis that decoupling took place throughout the spatial universe at the same time, give or take a few years, then the CMB ‘photons’ received on Earth today from all directions should have the same temperature-wavelength. If the Earth had a relative velocity in a given direction, this would cause a Doppler shift of the wavelength this direction with respect to the opposite direction.

If this description is essentially correct, would a model of an expanding homogeneous and isotropic universe infer any meaning to the magnitude and distribution of the relative velocity of observable galaxies with respect to the CMB frame?
oldman
#24
Nov6-08, 08:42 AM
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Quote Quote by mysearch View Post
....As I understand it, the CMB rest frame is not actually making any comparison to a relative velocity to radiation ...
Yes, you're correct. MeJennifer had the wrong end of my stick here. I wasn't being careful enough. In the consensus model the CMB radiation provides a personal reference frame of rest for any observer --- simply a frame in which she/he observes this radiation to have the same-temperature black-body spectrum no matter in what direction it is being observed. In the consensus model observers at rest in such frames separate from each other --- their (carefully defined) 'proper distance' apart increases with time --- they are said to partake of the 'Hubble flow' as the model universe expands in a GR way.

So there is no single absolute frame of rest, no universal ether, just different frames for different folks.
If the Earth had a relative velocity in a given direction, this would cause a Doppler shift of the wavelength this direction with respect to the opposite direction.
It actually does.
... would a model of an expanding homogeneous and isotropic universe infer any meaning to the magnitude and distribution of the relative velocity of observable galaxies with respect to the CMB frame
Here you touch on very recent observations of systematic motions whose origin has been tentatively attributed to happenings outside our observable universe. But this is altogether off-topic for this thread -- I don't have a reference, either.

I'm off to the warm sandy shores of the Indian Ocean for the weekend.
marcus
#25
Nov6-08, 09:05 AM
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Quote Quote by mysearch View Post
...
If this description is essentially correct, would a model of an expanding homogeneous and isotropic universe infer any meaning to the magnitude and distribution of the relative velocity of observable galaxies with respect to the CMB frame?
Probably safest not to talk about CMB frame. The word frame suggests a rigid reference frame with a fixed distance scale---technical connotations engrained in our minds from special relativity. The CMB does provide a universal criterion of rest, being at rest with respect to CMB (no doppler hotspot) or with respect to the matter of the early universe. But two things both at rest relative CMB can nevertheless have the distance between them increasing. So in the orthodox sense of frame, they don't belong to the same frame. I think you understand this and were just talking about motion of galaxies with respect to CMB.

so your question makes better sense if you just leave off the word frame at the end.


If I understand your question right, I think the answer is yes. Note that inferences in cosmology are rarely absolute or final---astronomers are constantly refining their ideas.

When people were talking some 10-15 years ago about the Great Attractor located in the direction of constellations Hydra and Centaurus in the southern hemisphere, but too far away to see or too obscured by intervening stuff, what they were really observing was a collective DRIFT of various clusters of galaxies relative to CMB rest.

There was another result like that recently, but not yet confirmed, purporting to have identifed a statistical drift in a sample of over a thousand galaxies, again relative to CMB rest.

Each galaxy has its own individual motion relative CMB, though in general it's difficult or impossible to determine (except for the radial component of motion relative to us---some small deviation from the expected redshift, receding just a bit too fast, or not fast enough, to fit the overall pattern.)

With nearby clusters, such as Virgo cluster, it's easier to gauge than for more distant. And of course for our galaxy, and our local group of 10-20 galaxies it is even easier. So there are published figures on the various individual motions----speed and direction----for some mostly nearby things. These are comparatively SMALL (a few hundred km/s) and not part of the expansion process.

For example the solar system is going 380 km/s relative to the CMB---in the direction of Leo. We know that because there is a large doppler hotspot in that direction. A few microkelvin hotter CMB in that direction and a few microkelvin colder spot in the opposite.

This 380 km/s is the composite of our orbital speed within the galaxy and the galaxy's own collective motion, which is about 500 km/s in a different direction closer to Centaurus (actually a small constellation near Centaurus called Crater which means wine-cup.)

Virgo cluster (the nearest really big cluster) is also going some 500 km/s in a Centaurusish direction, as I recall----as well as they can determine. All these motions are different from Hubble law recession and they are calculated relative to CMB rest.

If all galaxies were perfectly stationary relative to CMB then they would still have their recession redshift, but they wouldn't have these small individual deviations. But they seem to have some mostly random deviations. And when people study these individual deviations from perfect Hubble law recession, they can come up with inconclusive suggestions of some sort of large scale coincidence or DRIFT. I mentioned that earlier. It is the sort of thing which is a teaser. If it is confirmed then it suggests there is a departure from uniformity and a concentration of mass in some direction, hitherto beyond our ken----like the fabled Great Attractor. It could be just a percentagewise minor blip in the otherwise uniform distribution but a small percent increase density over a very large volume could make a big gravitational effect on us, and explain a drift (relative to CMB rest) of ours and surrounding galaxies.

So your question is, does cataloguing individual motions relative CMB let us infer any meaning? And the answer is YES, if it is confirmed to be not perfectly random but has some overall drift direction then it it would force us to modify our assumption of uniformity, and let us infer an estimated size and direction of non-uniformity. This then if confirmed would represent early universe structure and would have to be included in the models of early universe structure formation. What's a plausible explanation for why this deviation? Is it compatible with the prevailing inflation scenarios? etc etc.
marcus
#26
Nov6-08, 09:16 AM
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Quote Quote by oldman View Post
Here you touch on very recent observations of systematic motions whose origin has been tentatively attributed to happenings outside our observable universe. But this is altogether off-topic for this thread -- I don't have a reference, either.

I'm off to the warm sandy shores of the Indian Ocean for the weekend.
Damn! That is so enviable! Enjoy the warm sandy shores. Where do live the rest of the time?

I'll get a reference for that recent study that purports to detect a drift, in a sample of over 1000 galaxies.
mysearch
#27
Nov7-08, 07:41 AM
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First thanks for all the background information:
But two things both at rest relative CMB can nevertheless have the distance between them increasing.

I agree. However, I wanted to outline a conceptual model to clarify a few ideas and what I meant by a CMB frame. Imagine a very large, but arbitrary, spherical volume of space defined by a radius [r]. On the surface of this conceptual spherical volume, there are a number of distant galaxies, which have no velocity with respect to CMB. As such, I believe they define a volume of space, which will expand in-line with the scale factor, i.e. a(t).

This volume of space contains baryon matter, radiation, cold dark matter and dark energy. As the volume expands, the energy density of the first three dilutes, but dark energy remains constant. The increasing relative % of dark energy causes the rate of expansion to accelerate. However, our reference galaxies maintain their relative position while remaining at rest with respect to the CMB. As such, they seem to provide a frame of reference within an expanding universe. So attempting to answer my own original question:
If this description is essentially correct, would a model of an expanding homogeneous and isotropic universe infer any meaning to the magnitude and distribution of the relative velocity of observable galaxies with respect to the CMB frame?

Within the context of the conceptual homogeneous model outlined, it would seem that any velocity with respect to CMB is an anomaly, which corresponds to the observation that the universe is not uniformly homogeneous, at least, on the local level. As such, most galaxies, solar systems and planets will have been shifted from CMB rest due to localised distributions of gravitational matter, while still basically conforming to the recessional velocity of an expanding universe?
marcus
#28
Nov7-08, 10:13 AM
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Quote Quote by mysearch View Post
...
Within the context of the conceptual homogeneous model outlined, it would seem that any velocity with respect to CMB is an anomaly, which corresponds to the observation that the universe is not uniformly homogeneous, at least, on the local level. As such, most galaxies, solar systems and planets will have been shifted from CMB rest due to localised distributions of gravitational matter, while still basically conforming...
That puts it very clearly. This agrees with what I think is the mainstream cosmic picture and I accept it pretty much verbatim without reservation*.

What you are saying is most motion of macroscopic objects relative CMB is due to stuff falling. Localized unevenness, differences in primordial density, would have started stuff falling from low-density regions to higher.

To the best of my knowledge (and I hope some more knowledgeable person can correct me if I am wrong) most of that kind of individual object motion would have arisen that way and must continue to arise that way.

There are violent events which can impart motion by other means, but they can't compete with simple falling to explain most motion. Examples are supernovae, and gravitational slingshot events that somehow manage to hurl compact objects and may even give them enough speed to escape from their home galaxies. But I think overall by far the most common source of anomalous motion must be simple falling.

One of the most interesting questions is: How did the primordial unevenness in matter distribution originate in the first place? One conjectured explanation traces non-uniformity to "quantum fluctuations"----I put that in quotes because I'm not sure by what mechanism the quantum fluctuations would have become actualized as real largescale non-uniformity. Who collapsed the wave function? Why isn't the cat still in two inconsistent conditions? Like everybody else I have read various discussions of this but I remain uncertain about the origin of primordial structure i.e. density variations.

If you ever read the Roman author Lucretius he traces it all back to an accidental Swerve, an accidental deviation from uniformity that triggered the whole blarsted coagulation. Like us he dimly perceived that there was a problem and like us he hopefully offered his suggestion In the subsequent 2000 years we apes have made enormous strides and yet certain things remain unresolved.

*apart from a few quibbles about wording.
RandallB
#29
Nov9-08, 12:53 PM
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Quote Quote by marcus View Post
When people were talking some 10-15 years ago about the Great Attractor located in the direction of constellations Hydra and Centaurus in the southern hemisphere, but too far away to see or too obscured by intervening stuff, what they were really observing was a collective DRIFT of various clusters of galaxies relative to CMB rest.

There was another result like that recently, but not yet confirmed, purporting to have identifed a statistical drift in a sample of over a thousand galaxies, again relative to CMB rest.
I hope they are still talking about the Great Attractor and put these new observations in context with the GA.
I’ve seen but a few comments on this new “collective DRIFT” but they have not been clear on direction or scale of the new observations.
I can only assume it must be confirming the “Great Attractor” observations in effect and direction or they would have announced any conflict with it.

As I understand the Great Attractor observation is based on our Local Group (Less than 50 Galaxies). Do you know if the 1000 galaxies used in the “Collective DRIFT” observations included our local group and nearby.
Or might they have focused on an area away from us and Hydra-Centaurus area in an attempt to find a triangulation to the Great Attractor.
That strikes me as a good approach, picking a large area away from us and the GA should give a different angle pointed towards the GA if it is there. But if it gives a vector to a new Great Attractor Great for that distant area parallel to the vector we have to our GA, it would iindicate some form of universally Drift at an even larger scale of an asymmetric universe. That would be a new form of a non-homogenous indication.

Hope you find that reference,
some detail on how it compares to the LG GA observations should be interesting.
marcus
#30
Nov9-08, 02:28 PM
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Quote Quote by RandallB View Post
...
Hope you find that reference,
some detail on how it compares to the LG GA observations should be interesting.
Hi Randall, thanks for reminding me! I did say I would get links to the two papers by Kashlinsky et al that claimed to see indications of a "dark flow".

It wasn't confirmed (so far anyway) and flaws were found in their analysis, which Ned Wright lists here:
http://www.astro.ucla.edu/~wright/dark-flow-errors.html

He also gives links to the two papers, so I don't need to. His conclusion warns that their conclusions cannot be trusted.
oldman
#31
Nov17-08, 07:31 AM
P: 622
When trying in a dim sort of way to comprehend the consensus model of the universe --- the one Marcus is trying to build a 'same page to get on' about --- I've found it useful to imagine toy models in which extreme circumstances prevail. Sometimes this leads on to questions that I don't have answers for. Hence this further Jazz.

I find the vastness of the model observed universe quite unimaginable, with its remote boundary now at a proper distance of about 46 million light years. This distance is imposed by the tiny Hubble constant, presently about 2.4 x 10 ^ -19 per second.

Since I have no idea at all what determines the numerical value of the Hubble constant, I feel free to imagine a toy model that expands as absurdly fast as I like. Why not? --- Alan Guth did just this!

Sometimes I find it more comfortable to imagine a table-top model of the observed universe, choosing a Hubble constant of the order of 10 ^ +7 per second. The observed-universe boundary is then only a few tens of meters away. I also like to imagine the Hubble constant to be eternally constant, so that there are absolutely no gravitational tidal forces that can distort the shapes of everyday objects like myself, my steel ruler and mechanical tick-tock clock with which I set up coordinates and explore simple physics, as ruled by SR with the ordinary value for c.

Just as I begin to think that in this tabletop universe I could ignore such an absurd rate of expansion of the universe around me, and expect ordinary physics to prevail, I remember that in this toy universe extreme redshifts would occur for light signals transmitted between points of spacetime. I imagine that large redshifts would in this case affect the workings of atomic and particle physics, e.g. interparticle interactions.

Which brings me to the general question: GR assumes that local physics obeys the same covariant laws with the same c throughout spacetime. Should one imagine an upper limit for the Hubble constant in an expanding universe, to ensure that the expansion leaves enough local 'room' in spacetime for the workings of say, QED or QCD as we understand these theories, to remain perceptibly unaffected? Or is local physics likely to be somehow changed by very rapid expansion, as occurs in, say, the inflationary scenario?
marcus
#32
Nov17-08, 08:20 AM
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Quote Quote by oldman View Post
.
...I find the vastness of the model observed universe quite unimaginable, with its remote boundary now at a proper distance of about 46 million light years.
three orders of magnitude


This distance is imposed by the tiny Hubble constant, presently about 2.4 x 10 ^ -19 per second.
one order of magnitude

... no idea at all what determines the numerical value of the Hubble constant,
General Relativity. It's determined dynamically by GR. The Friedmann eqns derive from GR, and the first Friedmann eqn (which dates from around 1922) specifies the changing numerical value of the Hubble parameter. Indeed, the square of that parameter constitutes the righthand side of that equation. Calling it a constant was an unfortunate misnomer. You can see from the equation that the square is proportional to density, so it has to decline as the universe thins out. It has always been known not to be constant.

... I also like to imagine the Hubble constant to be eternally constant, ...
Then you are choosing to take leave of General Relativity. You'll be needing a new theory of gravity. Hope you find one and it goes all right.
oldman
#33
Nov17-08, 11:56 PM
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Quote Quote by marcus View Post

General Relativity. It's determined dynamically by GR. The Friedmann eqns derive from GR, and the first Friedmann eqn (which dates from around 1922) specifies the changing numerical value of the Hubble parameter. Indeed, the square of that parameter constitutes the righthand side of that equation. Calling it a constant was an unfortunate misnomer. You can see from the equation that the square is proportional to density, so it has to decline as the universe thins out. It has always been known not to be constant.......

I'm not clear what your comments about orders of magnitude mean. Have I got numbers wrong, perhaps? Probably.

But let me clarify why I said that I have no idea what determines the numerical value of H.

H is, via Friedmann I, and for a spatially flat geometry, expressed in terms of two variables: namely mean density and Lambda. We have some idea of what the density of our universe is, but no idea at all of what determines Lambda. To match the presently observed H and flat geometry we accept an appropriate value for Lambda, perhaps calling it dark energy. We have no idea why Lambda, and hence our H, have the values they now do. So I still do have no idea why H is what it is!

If I choose to imagine an absurdly large value for H in my toy model, I am in effect choosing a Lambda to suit my fancy. Since there is a 10 ^ 120 discrepancy between the postulated value of Lambda and its rationale as vacuum energy, what's wrong with this liberty?

I take your point about H varying. I was trying to exclude from consideration the tidal forces that are caused by the rate of expansion varying. These might be confused with non-existent forces that are often erroneously attributed to expansion itself.

Thanks muchly for your good wishes about my getting hold a new theory of gravity. I'll remember them kindly. But for the moment I'll stick with GR!


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